In a free-electron laser an electron beam interacts with a periodic electromagnetic structure to amplify electromagnetic waves. A short history of the device, a summary of major accomplishments, and a tutorial single particle free-electron laser physics analysis are presented.

After an introduction to the main ideas of high order harmonic generation, a review of recent experimental results is presented. Calculations dealing with the effect of intermediate resonances and of the ion also are reviewed. Finally, the ideas behind inversionless amplification are briefly discussed.

CO2-lasers are widely used in industrial applications. The increasing number of demanding applications as well as market needs are defining several trends in laser development. For welding and cutting of heavy steel sections as well as high-speed surface treatment, lasers with high output power are needed, well above 5 kW, which is the maximum for most applications now. Increasing the beam quality is a second trend which can be observed. High beam-quality systems are essential for sophisticated applications in the field of quality controlled and precision processing such as fine cutting, high speed cutting, and high speed welding. Along with these technical considerations the market development is demanding CO2-lasers, which can be produced at much lower prices compared to conventional systems in order to be competitive worldwide and to open new applications and markets for CO2-lasers. In order to meet the physical and technical requirements of these trends, a couple of basic investigations in the field of excitation and resonator physics as well as new technical designs are necessary. An overview on the respective activities of the ILT along with examples of technical designs are given in this paper.

This paper reports on a new type of Q-switched high power CO2 laser constructed for applications in materials processing. The working principle is described and the experimental results for pulse repetition rates at f equals 11 kHz are discussed.

A new design for a rf-excited CO2-laser uses coaxial electrodes. A high frequency blower located at one end of the electrode system generates a fast axial gas flow between the electrodes. The gas flow is reversed after passing the blower and moves back to the other end of the electrode system along the outer electrode, that carries a water-cooled heat-exchanger. So the gas reaches the other end of the electrode system with lower temperature and after a second reversal it again enters the cooling system. This kind of geometry then allows a very efficient cooling. Since the plasma has a hollow cylindrical shape, the extraction of radiation is not trivial. Several possibilities are available, e.g., a multipass-zig-zag-beam geometry. A theoretical estimation shows that this geometry, where the electrode system, gas flow and heat exchanger arrangement, and the resonator are integrated in a very rugged module, allows the user to obtain -- at least theoretically -- a beam power of 15 kW with a length of approximately 1 meter and an overall diameter of 70 centimeters.

A low pressure (23 mbar) CO2 laser with fast mechanical Q-switch (f <EQ 20 kHz) provides pulses [t(0.5) approximately equals 200 ns] at several wavelengths simultaneously. The special design of a three-mirror cavity allows the oscillation of up to six arbitrarily selected wavelengths of the same vibrational band.

Laser-induced chemistry has received much attention in the past few years. The economics of such applications are dominated by the costs of photons and the quantum yield of the specific reaction. For a typical multiple-IR-photon process the quantum yield can be as low as 10-4 which emphasizes the importance of reducing the cost of laser photons. Based on 1982 technology, CO2 TEA laser operating costs were approximately $100/watt per year for a laser with an electrical efficiency of 6% and an average power of more than 100 kW. Capital costs dominated the energy cost as well as the maintenance and labor costs. At the South African Atomic Energy Corp. we have been involved in the development of high pulse frequency, high average power TEA-CO2 lasers for the application in the field of laser-induced chemistry. Much of the attention, however, has been focused on the application to separate the isotopes of uranium via a multiwavelength infrared irradiation scheme. The progress that has been made towards the establishment of CO2-lasers and laser chains for industrial use has been quite outstanding.

Performance characteristics of a high power Q-switched CO2 laser based on a fast gas flow system have been investigated. A combination of a fast rotating chopper and an intracavity telescope as a Q-switching device has been applied to a fast axial flow, dc- discharged 1.5 kW continuous wave CO2 laser. The laser delivered 480 W average output power at 12 kHz pulse repetition rate on a free-running basis. Peak power of each Q-switched pulse is approximately 100 kW, and the pulse duration is 250 nsec. An analytical model describing the unique characteristics of the CO2 laser medium and of the rotating chopper Q-switching technique has been developed and applied to the experimental results. Good agreement was obtained between the theory and experiment for various pulse parameters such as peak power, pulse duration, and pulse buildup time. A method to improve the pulse characteristics using a fast axial gas flow system is proposed.

Microwave excitation of laser gases allows the deposition of high excitation energy densities into an active medium of relatively large diameter and volume. To take advantage of low cost commercially available generators a microwave frequency of (upsilon) equals 2.45 GHz was chosen. An oscillator amplifier system with TEM00 beam quality and tunable wavelength is used in order to obtain a cw laser output of maximum PL equals 7 kW. The output of a dc discharged, diffusion cooled CO2 laser oscillator operating in the TEM00 mode with an output power of 265 W (cw) and an average power of maximum 140 W in the pulsed mode is fed into an amplifier consisting of 24 modules, excited by two magnetrons of 2.7 kW power each. The discharge diameter of an amplifier module is d equals 0.051 m and the active length for one discharge module is 1 equals 0.3 m, yielding an active gain path of Lakt equals 7.2 m. In the pulsed mode the amplifier delivers a peak power of up to 1.9 MW at a repetition rate of 10 kHz.

We have developed a compact kW-class CO laser excited by transverse-rf-discharge. The discharge and output characteristics of the rf-excited CO laser were investigated experimentally. Six gas conditions (pressure, temperature, flow speed, CO-, N2-, and O2-concentration) are chosen as the experimental parameters. The optimum values of them to maximize the output power are 50 Torr, 200 K, 17 m/s, 5%, 12%, and 0.4%, respectively. The maximum output power obtained is 1.3 kW with the electrical conversion efficiency of 27% and the slope efficiency of 39%.

The use of radio frequency discharges for excitation of two types of carbon monoxide laser is discussed, with emphasis on the attainment of both good beam quality and operation at relatively high temperature. The first laser type uses fast axial flow in circular cross-section discharges, producing power output comparable to similarly dimensioned carbon dioxide lasers at cryogenic temperatures. Problems with obtaining good beam quality in this type of laser, related to the wavelength and high mass flow requirements, are identified. The second laser type uses stationary gas in a slab waveguide arrangement, and has been shown to operate with good efficiency nearer to ambient temperature than the fast flow device. Good beam quality is being sought by the use of waveguide-unstable resonators.

A gasdynamically cooled CO-laser with a dielectrically stabilized rf-discharge in the subsonic region is described, and laser performance data are presented. The laser is based on the well proven design realized several years ago in a 1-kW-laser at DLR Stuttgart. The dimensions have been scaled up to enable laser output powers up to 5 kW. The laser is run in a blow- down gas system which allows operation times of about 10 seconds. At present a maximum output power of 3.2 kW can be achieved.

A simple and compact computer code for the output beam mode analysis of the CO laser has been developed. The theoretical model based on the asymptotic expansion is converted to the computer model, which is divided into smaller structural modules. The program is described in C language in order to be compiled and executed by PCs, Mackintoshes, and workstations. The beam mode profiles under the multiline operation and misaligned cavity conditions are analyzed.

By insertion of a fast mechanical Q-switch into the resonator, continuous discharge CO2 lasers can yield high peak power pulses at multi-kHz repetition rate. First experiments have been done with diffusion-cooled low-pressure CO2 lasers. For this type of laser the pulse energy can only be increased by increasing the length of the active medium. A limit is given by the onset of uncontrolled self oscillation which prevents regular Q-switched operation. Single pulse energies can apparently not exceed 30 mJ at 250 ns pulse duration for this type of laser. Fast gas-flow convection-cooled laser discharges allow us to increase the stored energy by increasing diameter and pressure of the active medium as well as the electrical power density. We present the results of Q-switching of a 5 kW industrial laser. Our Q-switch is scalable in optical power. It is based on a fast chopper and a conical mirror. In some experiments we tuned the laser over a wide range by a diffraction grating. The influence of gas pressure and mixtures as well as discharge parameters has been studied. Single pulse energies of 100 mJ have been found, limited by the electrical input power density.

A laser kinetics code is developed to simulate the performance of a dc excited fast axial flow CO2 laser module. Being a five temperature model, the simulation consists of a set of equations for the relevant vibrational mode temperatures, the intra cavity radiation intensity, and the ambient gas temperature. Input electron excitation rates are computed by solving the Boltzmann equation for a given laser mixture and corresponding reduced electrical field strength E/N (N: gas density). The latter parameter is obtained experimentally by means of floating Langmuir probes. The significant population densities of vibrationally excited states in CO2 laser discharges necessitate taking into account the effect of superelastic collisions on the electron energy distribution function (EEDF). The calculated dependence of the laser output power on discharge current and pressure is in good agreement with the experiment.

The new kinetic model of CO laser is developed. The basis of the model is multiquantum vibrational exchange rate constants given by Billing. The full Billing model of multiquantum VV exchange gives rise to satisfactory coincidence with experimental data on vibrational distribution function without any fitted parameters. Computer models of vibrational kinetics for CO containing mixtures are currently widely used in the analysis of problems of highly nonequilibrium vibrational excitation and in investigations of CO lasers. Until now only the single quantum VV exchange models were considered. For single quantum processes CO(v) + CO(u) yields CO(v - 1) + CO(u + 1) the rate constants (RC) are usually calculated using expressions, based on the first order perturbation theory assumptions. The parameters of well known theoretical expressions are fitted thereafter to get the magnitude and vibrational quantum number dependence of RC, measured experimentally. The RC, extended in this way to VV exchange of highly excited molecules, grow rapidly with v,u and exceed gas kinetical RC at rather low v,u > 7 for quasi-resonance exchange. Thus the validity of the first order perturbation theory expressions breaks and vibrational kinetics models using these RC become doubtful for relatively low levels. Nevertheless, these RC were widely used in practical calculations of CO laser kinetics. Their justification was in satisfactory agreement with measurements of vibrational distribution function (VDF), available in the literature.

Our center has developed a new type of commercially available high-power (up to 5 kW) multichannel diffusion cooled waveguide industrial CO2-lasers with ac-pumping (MTL). The physical principles and the results of scientific investigations of these lasers have been published. Such a laser has a parallel bunch of gas-discharge laser tubes placed between two flat mirrors. The laser generates a large-aperture multibeam. The components of this multibeam may be non-synchronized, but there exist the optical methods of their synchronization and aperture-filling. The main advantages of the MTL are: relative simplicity of their construction, low cost of production and maintenance, high level of compactness. For example, the MTL-4 model (4 kW rated power) has a 60 kg, 2 X 0, 2 X O, 2 m laser head, 3 nl/h He consumption and 12% plug efficiency.

In the optical resonators of industrial CO2-lasers there exist stagnation zones with unexcited working mixture. In these regions the beam is partially absorbed by carbon dioxide; this results in variation of the laser beam energy and spectral characteristics. In this connection, of practical interest is the study of the unexcited mixture heating stability under various procedures of heat removal: gas mixture pumping, heat conductivity, or natural convection. For the optical thermal instability under consideration, there is an effect of thermal runaway known in chemical burning theory and emerging under self-acceleration heat release, when it cannot be compensated by heat removal.

A one-dimensional theoretical model has been developed to simulate the amplification and oscillation processes in a transverse electrically excited CO2 laser with gas flow longitudinal to the optical axis. It allows us to describe the complete beam generation process and it is able to solve specific problems of gas discharge or amplification performance. In this paper the model is used as a support for the understanding of special properties of longitudinal flow CO2 lasers that strongly affects the power performances. Particularly the uncertainties in the discussion of experimentally achievable coefficients of small signal gain are shown.

Fast axial flow rf-excited discharges ((nu) rf equals 13.5 MHz) in tubes with rectangular cross sections and plane electrodes have been investigated. Probing the tubes on-axis the dependence of small signal gain as well as saturation intensity on the input power density is identical for all the different cross sections. Under equal conditions the small signal gain profiles revealed significant decrease in the vicinity of the side walls with increasing tube dimensions. This is attributed to the change of discharge behavior with increasing interelectrode discharge length. It was found that the critical pressure for the onset of filamentation is reciprocal to this dimension. With this geometrical dependency the laser power of axial flow rf-excited discharges is not volumetric scalable in a straightforward way.

Axial-fast-flow CO2 laser was investigated numerically taking into account turbulent and convective cooling of the gas medium. The model developed includes a description of the vibrational kinetics and the gas motion. The calculations were made for two real 500 W and 1200 W CO2 lasers, respectively. The results obtained show a good agreement between the experimental and theoretical data.

An output power of 2 kW is attained with a compact coaxial slow-flow CO2 laser. The resonator consists of two toric copper mirrors, which are tilted a small amount. These mirrors form an unstable resonator in azimuthal direction, from which two laser beams are extracted with high efficiency through a coupling aperture in one of the mirrors. The beam divergence of a single beam is nearly diffraction limited in the radial and azimuthal direction.

In order to achieve a compact industrial fast axial flow CO2 laser delivering an output power of a few kilowatts with a high beam quality, the authors have investigated the behavior of an amplifying medium excited by an ac discharge also called `silent discharge.' The external electrodes discharge tube and this ac power supply have been developed. The power supply was working in a resonant mode within a frequency range of 95 kHz to 125 kHz and showed that the power transmitted to the load can be modulated by frequency modulation. The dependence of the capacitive coupling between the power source and the discharge load on the electrode geometry are studied. The dependence of the gas temperature on the electrical characteristics of the source such as voltage, current, phase, and injected power are displayed. In this paper we show that for a gas temperature of 200 degree(s)C the maximum input power density was limited to 11 W/cm3. In an attempt at a laser application, an efficiency of over 15% was obtained with a laser output power of 800 W for an electric power of 4,800 W.

A fast axial flow high power CO2 laser excited by dc discharge has been developed. An aerodynamic technique was developed for achieving homogeneous discharge of power density up to 6 W/cm3 in 5.5 cm diameter tubes. An output power of 10 kW was obtained with an unstable two-folded resonator with a magnification of 2. The laser is made of 12 active mediums of 5.5 cm diameter and 60 cm length and the electrical discharge efficiency is 17%.

A multikilowatt operation of a multijoule transversely excited atmospheric CO2 laser driven by an all-solid-state exciter has been demonstrated. The exciter consists of a high- voltage pulse generator employing a newly developed semiconductor stack switch, 48-series and 2-parallel insulated gate bipolar transistors, and a two-stage magnetic pulse compressor. The maximum average laser power of 4.6 kW was obtained with an overall efficiency of 10.5% at a repetition rate of 550 pps. The maximum repetition rate of 1100 pps was attained with an average power of 3.4 kW and an overall efficiency of 7.4%.

For a dc self-sustained discharge adapted for excitation of a transverse-flow 1.2 kW CO2 laser stable operating condition in a CO2/N2/He equals 1/9/15 mixture at static pressure of 30 - 40 Torr and by flow velocity up to 70 m/s were obtained in the excited volume of 2.1 dm3 by the dissipated electric power density up to 9.5 W/cm3. It has been confirmed that the passivation and conditioning of electrodes influences strongly the long-term output characteristics.

The kinetics of chemical processes in the laser gas mixture is theoretically studied to explain its effect on power extraction of the transverse-flow 1.2 kW CO2 laser system. The reactions of chemical species with excited electronic states, ions as well as heterogeneous recombination, are included into the model. The chemical equilibrium strongly depends on the excitation conditions.

The small signal gain distribution along the flow is taken into account in the diffraction type analysis of the output properties of a stable multipass resonator designed to operate with transverse-flow cw CO2 laser. Experimentally obtained gain profiles for different laser excitation parameters are introduced to the numerical model of the resonator. The laser output characteristics are analyzed and compared with the experimental data.

The conditions for fundamental mode operation of the transverse flow CO2 laser designed for industrial applications have been investigated experimentally. The effect of the field limiting apertures on the laser output characteristics has been studied for three stable resonator configurations described by the different value of Frensel number, G-parameter, and active medium length. The results are compared with the predictions of the theoretical analysis concerning the diffraction properties of the stable resonator.

The strikingly small affect of the mixture degradation on the power extraction observed experimentally in the MLT-1200 laser has led to the examination of the time characteristics of laser output as a function of the gas mixture composition. Processes competitive to the working mixture degradation were considered and we found that the optimum concentrations of CO2 and N2 lie in the region 2 <EQ [CO2] <EQ 3% and 30 <EQ [N2] <EQ 40%, respectively.

In this work, a new grid was added to a grid corona discharge TEA CO2 laser. The new grid was mounted under the cathode and inside the laser tube. The effect of this new mesh on the uniformity of the glow discharge was investigated. Experiments showed that the new mesh extends the arcing limit of oxygen concentration up to 9.5%. In this study, the effect of the tube size and the electrode spacing was tested on the output and loading energy density in the medium aperture of 2.5 cm on the laser tube.

Plasma injection method has been used to stabilize the main electric discharge in the atmospheric high power cw CO2 lasers. In this work, an analytical perturbative method is introduced for solving Boltzmann equation for a general plasma. This method is applied to a specific plasma injection CO2-N2-He laser. Small signal gain and output power of the laser are calculated in the steady state as a function of gas mixture and pressure. Dependence of small signal gain and output power on the main discharge current also were considered.

Transverse electric atmospheric (TEA), or multi atmospheric (TEMA) lasers deliver intense short laser pulses of considerable energies. Recurrent high repetition rate pulse trains afford substantial average power levels. In a high rep-rate operation the gas flows across the cavity and is externally cooled to maintain a reasonably low temperature. The gas flow gear and heat exchanger are bulky and costly. In this work we present a repetitively pulsed TEA or TEMA laser that combines energy and peak power features in an individual pulse with the substantial average power levels of a pulse train in a thin layer of gas. Excess heat is disposed of, by conduction through the gas, to cooled enclosing walls. The gas does not flow. The method applies to vibrational transition molecular lasers in the infrared, where elevated temperatures are deleterious to the laser operation. The gist of the method draws on the law that heat conductivity in gases does not depend on their pressure. The fact lends unique operational flexibility and compactness, desirable for industrial and research purposes.

The discharge effects on gas flow lasers have been treated from both aspects of the activating features to the laser performance and of demerits by electrical heating and disturbances in the laser medium. The approach has been from two types of studies, namely on the continuous discharge in CO2 supersonic mixing electric discharge laser (EDL) and on the pulsed discharge in excimer laser. For CO2 supersonic mixing EDL, small scale experiments have been performed on the small signal gain coefficient and on the laser power, together with the numerical analysis by using the quasi-one-dimensional and vibrationally nonequilibrium equation system. Discharge effect is included by solving the Boltzmann equation for electron energy distribution function, and power extraction analysis also is carried out. By comparing the experimental results, various characteristics have been clarified on this type of supersonic discharge laser. As for the pulsed discharge, flow visualization experiments have been conducted in the model cavity of excimer laser, along with a numerical calculation on the one- dimensional Euler equation system by TVD approach. There have been three types of waves in the laser cavity, and Mach numbers of horizontally propagating main waves have been discussed from both numerical and experimental aspects.

In previous works a numerical study of the unsteady two dimensional flow in the cavity of an excimer laser has been presented. In this paper, the numerical method based on a finite difference scheme, associated with a flux transport algorithm (SHASTA-FCT method), is developed for computing the flow in a pulsed chemical laser. The gas mixture contains SF6 and the evolution of the ratio of specific heats (gamma) as a function of temperature must be included in the numerical approach. The elliptic shape of the lower electrode leads to a convergent divergent laser cavity and allows a better damping of pressure waves due to the non-uniformity of the energy deposition. From a comparison between numerical and experimental results on the evolution of the wall pressure at a given position it is possible to come up to the real shape of the energy deposition. In addition, the numerical simulation of the flow is carried out over two interpulse time to get the description of the flow field in a multidischarge configuration.

Theoretical and experimental investigations on the propagation, reflection, and attenuation of shock waves as they occur in excimer lasers have been performed. The numerical simulations have been carried out using a two-dimensional, unsteady finite difference scheme. The experimental setup is a piston driven shock tube with a rectangular cross section working in air at atmospheric pressure. The shocks were detected interferometrically as well as by means of pressure transducers. This shock tube allows us to investigate basic phenomena of shock diffraction which can be used to confirm the computational results in the range of weak shock waves. In particular, the influence of the shape of the wall contour on the reflection of shock waves has been investigated theoretically. The decay time of pressure and density perturbations differs for various wall configurations in such a way that short electrodes accelerate the attenuation as well as does a strong area increase in the vicinity of them. After each laser pulse there is a shock travelling into the laser channel. Experiments have been carried out on the reflection of this shock at a specially formed bend that is able to focus the shock into a muffling element.

A theoretical analysis is made to describe the influence of an active disturbing medium on the far-field diffraction pattern of an excimer laser beam. Results are in good agreement with experiences. The gas density perturbations induce amplitude and phase aberrations which are responsible for the degradation of the beam characteristics in the far-field.

The possibilities of a longitudinal N2 laser pumped by BIW have been demonstrated. The developed laser gives us the possibility of exciting a nitrogen with specific stopping energy of about 0.5 J/cm3 and of producing laser action in N2 with an output power up to 100 kW in pulse having a duration of 10 ns FWHM with excellent laser beam quality.

Depending on the mutual orientation of the output laser beam and direction of electric current in the discharge gap, two schemes of laser pumping by electric discharge are distinguished: longitudinal and transversal. Currently, most commercial pulsed lasers operate on the transversal scheme of excitation, with more powerful laser radiation due to higher stopping electrical power in the gas. The longitudinal scheme is used for producing a laser beam with more qualitative characteristics. In this case, due to the discharge gap length being longer than in the transversal scheme, the nanosecond electrical pulse with a higher amplitude is used. The value of the reduced electrical field strength E/P remains low for effective excitation and ionization of the gas. Typically the time of breakdown development and electric current growth is not enough for effective laser generation. We propose overcoming these shortcomings with the use of nanosecond gas discharge in the form of breakdown ionization wave.

The comprehension of the performance of pulsed gas lasers requires knowledge of several parameters which are time dependent and consequently difficult to measure. These parameters are mainly influenced by the two electric discharges which take place during the laser performance, namely by the ignition system and laser tube. The present work illustrates a new method to determine the most fundamental of these parameters, the current. This can be achieved exploiting only the voltage waveform on which much concealed information exists about all the time dependent parameters. The revelation of these parameters can be achieved by further elaboration of the waveforms of the high voltages. This elaboration, in our case, leads to finding the current and is described in this paper. The method is applied to a simple RLC circuit, to a doubling, and to a C-to-C circuit and it is delivered from experimental errors existing in other methods achieving the best accuracy to date.

Utilizing a pulsed beam of a Nd:YAG laser, hole-burning through the opaque cloud of products formed following the detonation of lead azide is demonstrated. The characteristics of the hole and of the expanding cloud are monitored in real time by a HeNe beam and by high- speed framing photography. The hole is carried with the cloud and propagates at a constant velocity of 0.5 - 2.8 km/s, depending on the time and location of burning. The hole-burning is a result of eliminating solid particles from the cloud. The reduction in the number and size of the particles is monitored by scanning electron microscopy of the deposits formed on a substrate following the detonation. The application of a laser to burn a hole in the detonation products from a solid explosive is demonstrated for the first time. This technique may serve as a method for flow visualization in an aerosol medium.

The effects of the supersonic nozzle shape on the performance of CO2 gasdynamic laser are analyzed by solving time-averaged two-dimensional Navier-Stokes equations coupled with the vibrational relaxation equations for a CO2 - N2 system. The solver is based on a finite difference method with an implicit high accuracy TVD scheme. Numerical simulations are carried out for various types of supersonic nozzles. The performance is estimated with the indicator of the small signal gain coefficient. The results show possibility of improvement for small signal gain coefficient by nozzle shape modification and suggest a suitable nozzle shape.

It is a serious task to control the flow pattern in the cavity of a chemical oxygen iodine laser (COIL) in order to improve the performance of the facility. The major task is to mix the iodine with activated oxygen uniformly. We performed the computer calculation simulating the actual test plant. Results of the calculation show a uniform concentration rate of iodine corresponding to the actual power plant.

In order to increase high power closed cycle CO2 laser performance, an experimental investigation of the velocity field in a cylindrical glow discharge tube was carried out. The axial velocity profile was measured (without electric discharge) using a laser Doppler anemometry technique at several sections of the discharge tube. The turbulent compressible flow has also been theoretically investigated by means of the PHOENICS code and comparisons of numerical results with experimental data show a reasonably good agreement.

Emission spectra from a high-current line plasma produced by means of a formed ferrite plasma source have been measured in the 1200 - 2900 angstrom spectral region. An observation of the plasma expansion with the formation and the propagation of a blast wave (1.8 km/s) is presented. The possibility of a peak intensity shift of VUV radiation to shorter wavelengths is discussed.

High repetition rate excimer lasers are expected for wide industrial application. The power of excimer laser, however, decreases rapidly in a higher repetition rate operation. Shock or acoustic waves, which are caused by the periodic pulse discharge, may limit the repetition rate of an excimer laser up to 2.5 kHz. Such waves cause inhomogeneity of gas density in the discharge region of the excimer laser. In high repetition rate operation this inhomogeneity remains at the next discharge. Arcing may be generated by this inhomogeneity and the homogeneous excitation of the laser gas is obstructed. Although these phenomena have been reported, the research for the effects of shock waves has remained insufficient. And the relation between these shock waves and discharge phenomena has not been clarified. To resolve this problem, we developed a scaling model chamber of a UV preionized excimer laser cavity with windows for flow visualization. We report the first result by using this model and Schlieren technique in a pure helium gas case. In our experiment three types of shock waves are found in the discharge cavity. Those shock waves are generated from the boundary of the main discharge area, from sparking pin gaps, and from the main electrode surfaces. In this study we focus on the effect of xenon gas on the generation and the propagation of shock waves. Components of the Xe-Cl excimer laser gas are helium, xenon, and hydrogen chloride. In those gases xenon has the largest molecular weight of 131.29. So we conclude xenon plays an important role in the shock wave propagation and in discharge phenomenon.

Supersonic expansion of an He fast discharge excited plasma is shown to produce the HeII 164.0 nm line by recombination. Results related to the spectrum, the time profile of the emitted radiation, and the effect of the backing pressure are presented. The study is of interest because of the possibility of getting amplification in the HeII line at 164.0 nm.

The overview summarizes past and present research on short-wavelength chemical lasers (SWCL) with special emphasis on recent advances in the field. Guidelines for developing SWCL and the most promising candidates are presented. It is concluded that premixed, pulsed chemical lasers may have some inherent advantages over their continuous-wave counterparts for operating a long sought after viable SWCL.

We demonstrated a high energy delivery of a pulsed wavelength-selected HF chemical laser by fluoride glass fibers (core/cladding equals 450/500 micrometers ). The optical energy of 19 mJ in a 540 ns pulse was successfully delivered with a peak intensity of 22 MW/cm2 at the exit core surface. We have also theoretically investigated the operational characteristics of the pulsed chain first vibrational overtone HF chemical laser using a computer code. The model used can describe simultaneously both the fundamental and overtone oscillations. The higher overtone output energy of 2.73 J/l can be obtained with a gas mixture consisting of F2/H2/He equals 10/4/786 (Torr) by successfully suppressing the ASE.

A compact, repetitively pulsed HF chemical laser, built with a gas recirculator loop to cool and process the gas mixture is described. A simple corona phototriggered discharge is used to produce vibrationally excited HF molecules at typical pressures of 90-150 Torr. This laser produced over 5 J per pulse in single- shot operation, and a maximum average power of 500 W. at a repetition rate of 110 Hz. The electrical efficiency is 3%.

The B yields X electronic transition of the iodine monofluoride (IF) molecule is a well known potential system for a chemical laser operating at visible wavelengths. By using the reaction between fluorine atoms and excited atomic iodine, IF(B) was produced in a supersonic flow. F and I atoms are produced in a combustor and excited I atoms are produced by mixing excited O2(1(Delta) ) to the expanded supersonic flow of combustion products. Chemiluminescence of IF was observed at a pressure ranging from 5 to 10 Torr. Experimental spectra have been recorded and compared to synthetic ones. A population density of IF(B) close to 4 X 109 molecules/cm3 has been determined. No laser effect has been observed. A computer code has been developed and applied to the kinetic scheme. Calculated gain profiles, IF(B) population density, and laser power are in disagreement with the experimental values. Adding fluorine (F2) to the flow increases the measured population density of IF(B) by a factor of 10. This value seems to confirm the existence of the step I* + F2 yields IF2 in the production of IF(B).

Chemiluminescence was observed due to the reactions which take place in the supersonic flow of dissociated nitrogen trifluoride mixed with hydrogen. An intensive glow on the NF a yields X and b yields X transitions was identified. Concentrations measured of NF(a) and NF(b) states are high and permit hope for overcoming the lasing threshold in the supposed NF-IF laser.